FIG. 9 is a simplified diagram showing an exemplary conventional solar cell 40 formed on a semiconductor substrate 41 that converts sunlight into electricity by the inner photoelectric effect. Solar cell 40 is formed on a semiconductor substrate 41 that is processed using known techniques to include an n-type doped upper region 41N and a p-type doped lower region 41P such that a pn-junction is formed near the center of substrate 41. Disposed on an upper surface 42 of semiconductor substrate 41 are a series of parallel metal gridlines (fingers) 44 (shown in end view) that are electrically connected to n-type region 41N. A substantially solid conductive layer 46 is formed on a lower surface 43 of substrate 41, and is electrically connected to p-type region 41P. An antireflection coating 47 is typically formed over upper surface 42 of substrate 41. Solar cell 40 generates electricity when a photon from sunlight beams L1 pass through upper surface 42 into substrate 41 and hit a semiconductor material atom with an energy greater than the semiconductor band gap, which excites an electron (“−”) in the valence band to the conduction band, allowing the electron and an associated hole (“+”) to flow within substrate 41. The pn-junction separating n-type region 41N and p-type region 41P serves to prevent recombination of the excited electrons with the holes, thereby generating a potential difference that can be applied to a load by way of gridlines 44 and conductive layer 46, as indicated in FIG. 9.
FIG. 10 is a perspective view showing the front contact pattern of solar cell 40 in additional detail. The front contact pattern solar cell 40 consists of a rectilinear array of parallel gridlines 44 and one or more much wider collection lines (bus bars) 45 that extend perpendicular to gridlines 44, both disposed on upper surface 42. Gridlines 44 collect electrons (current) from substrate 41 as described above, and bus bars 45 which gather current from gridlines 44. In a photovoltaic module, bus bars 45 become the points to which metal ribbon (not shown) is attached, typically by soldering, with the ribbon being used to electrically connect one cell to another.
Conventional methods for producing the front contact pattern of solar cell 40 typically involve screen-printing both gridlines 44 and bus bars 45 using a metal-bearing ink in a single pass through a mesh with an emulsion pattern. Conventional screen printing techniques typically produce gridlines having a rectangular cross-section with a width W of approximately 100 μm and a height H of approximately 15 μm, producing an aspect ratio of approximately 0.15. There is no buildup of ink at the vertex of the bus bar and the gridline because the entire print has uniform thickness. However, the resulting structure is not necessarily optimal from a cell performance perspective however because the relatively low aspect ratio causes gridlines 44 to generate an undesirably large shadowed surface area (i.e., gridlines 44 prevent a significant amount of sunlight from passing through a large area of upper surface 22 into substrate 21, as depicted in FIG. 9 by light beam L2), which reduces the ability of solar cell 20 to generate electricity. However, simply reducing the width of gridlines 44 (i.e., without increasing the gridlines' cross-sectional area by increasing their height dimension) could undesirably limit the current transmitted to the applied load, and forming high aspect ratio gridlines using screen printing techniques would significantly increase production costs.
Although extrusion printing provides advantages over screen printing (e.g., higher aspect ratio gridlines), these advantages are partially offset by the peaked topography of each vertex 44-1B, i.e., where each gridline 44-1 crosses bus bar 45-1, which can make soldering the stringing metallization (i.e., the metal ribbon) difficult or unreliable. Each vertex 44-1B, as shown in FIG. 11(C), produces an uneven topography on the bus bars 45-1 that does not impact the cell performance, but can create a weak solder joint between the subsequently applied metal ribbon (not shown) and the top of bus bars 45-1 because there is insufficient solder to fill in the gaps between adjacent vertices 44-1B. That is, there is a potential problem with extrusion printing high aspect-ratio gridlines 44-1 on bus bars 45-1 in that, when a ribbon is subsequently soldered to the top of bus bars 45-1, the ribbon is unable to make a strong, broad area connection to bus bars 45-1 because of the ridges caused by gridline portions (vertices) 44-1B. One might consider printing the bus bar over the gridlines in an effort to better planarize the metallization. However, in the case of coextrusion printing, the present inventors have observed that, during the subsequent required firing process, if the gridline material resides beneath the bus bar, the bus bar metal tends to float and separate from the gridlines, causing an open circuit. Furthermore, the bus bar would still have an undulating surface which still might decrease the reliability of the soldered connection. Moreover, it is important to recognize that, once printed, the lower sections of the gridlines need to maintain their overall shape during the drying and firing process that sinters the metal into its conducting state. There is a characteristic shrinkage and densification of the metallization ink, however, the ink does not at any point become liquid which would enable it to flow, as this would cause the metal to ball up on the substrate causing breaks. For this reason, one can not simply planarize the metallization on the bus bars by briefly melting it, as it would damage the device.
What is needed is a micro extrusion printing method and associated apparatus for producing solar cells (and other IC devices that include similar bus bar/gridline arrangements) that facilitates the formation of extruded gridlines and bus bars at a low cost and addresses the problems described above. In particular, what is needed is a method for producing solar cells (and similar devices) that utilizes the benefits of extrusion printing, yet avoids the poor soldering problems caused by the peaked topography where the gridlines cross the bus bars.